Transgenic plants are a promising platform for the large-scale production of antibodies and other therapeutic proteins for treating serious and life-threatening conditions. The advantages of using plants, rather than genetically modified animal cells in bioreactors, include very low cost of goods, and safety, due to the inability of plants to be infected with mammalian viruses and prions. Production in tobacco, a plant not used in the human food chain, avoids concerns about adventitious presence of therapeutic proteins in food or feed. In addition, the amount of protein that can be produced per acre with tobacco far exceeds what is possible with a seed crop like corn. Costs of purification, however, are similar to that of proteins produced in mammalian cell culture. We propose adapting to plants a non-chromatographic extraction/purification technique previously used in bacterial expression of mammalian proteins. This technique uses the ability of elastin-like proteins (ELPs) to reversibly fall out of solution when heated to 30-40 oC. A fusion of ELP and Protein A or G, expressed in tobacco leaves, will produce a reagent useful for a simplified and economical method for extraction and non-chromatographic purification of antibodies from cell culture or plant tissues. As a further advance, we propose to use a self-cleaving intein that will allow a fusion of ELP and a therapeutic antibody to be separated cleanly in vitro. A combination of ELP and intein will be developed into a simple, economical and non-chromatographic method for purifying therapeutic proteins, including immunoglobulins and immunoadhesins, expressed in plants. Our model therapeutic protein in this project is a chimeric immunoadhesin that we have shown to neutralize anthrax lethal toxin. [unreadable] [unreadable] PUBLIC HEALTH RELEVANCE: Antibodies, and other complex therapeutic proteins, can be made much less expensively in plants than in animal cells, but the cost of purification is still high. This research may lead to a much more economical method for purifying therapeutic proteins. This will make possible forms of therapy that would previously have been too expensive. [unreadable] [unreadable] [unreadable]